1. Field of the Invention
The invention relates to an integrated positioning solution for global navigation satellite systems (GNSS) and especially relates to the dissemination of ephemeris, clock and navigation data for foreign navigation satellites via satellites of an operator's navigation system, and further relates to the seamless integration of ranging signals of other (foreign) GNSS into the operator's satellite navigation system.
2. Description of Background and Other Information
GNSS receivers (user segment) determine the pseudo ranges to satellites orbiting the earth via measurements of the pseudo signal propagation time between transmission at the satellite, in the physical timescale of the satellite and reception at the receiver in the timescale of the receiver. If the positions of the satellites and the offsets of the physical time scales of the individual satellites relative to the system time scale are known, the position of the receiver and the offset of the receiver time scale to the system time scale can be computed using pseudo ranges measured to several (at least four) satellites.
As the positions of the satellites change with time, the receiver needs a description of those changes as a function of time. Furthermore, the time stamp on the signals, which allows the measurement of the signal propagation times, refers to the individual physical time scales of the satellites and need to be related to the system time scale.
To combine the measurements to satellites of one system, the receiver must know the individual offsets between system time scale and physical time scale of all respective satellites. The information of the satellite position—or, more specifically, the position of the satellite's electromagnetic antenna phase center for each emitted signal—is given by the ephemeris; the information of the time scale offsets is given by the satellite clock parameters.
Both kinds of parameters are part of the navigation message transmitted by each individual satellite as an amendment of the ranging signal. These data are modulated on top of the navigation ranging signal. Only the combined use of all information—ranging signals to several satellites together with the respective navigation data (ephemeris and clock parameters and some other parameters describing the properties of the ranging signals)—make position determination of the receiver and hence navigation possible.
The ground-based monitoring network of a GNSS and dedicated data processing facilities (ground segment) are used to determine the parameters necessary for navigation. Beside the parameters essential for positioning like orbits and clocks, there are further parameters which enhance navigation or enable specific services, such as broadcast signal delay and integrity information. All parameters are determined and—as far as possible—predicted by the processing facilities and up-linked to the satellites for dissemination together with the ranging signal.
The description of the present invention is given from the point of view of an operator of a satellite navigation system (GNSS). It distinguishes between the “owned” system to which the operator or provider has usually full access, and “foreign” or “other” systems to which the operator has no access or only very limited access.
Different GNSS (like GPS, GLONASS, Galileo and COMPASS) are designed and are operated independently. This independence also refers to the determination of the navigation parameters. In principle, however, a user receiver is able to provide a better (in terms of more accurate and more reliable) positioning solution by combining measurements to satellites of different GNSS, if the time offsets of the systems and possible offsets and scaling factors of the geodetic reference systems can be determined from additional pseudo range measurements to a multitude of satellites.
The use of pseudo range measurements to satellites of different GNSS for an integrated (combined) position solution, where less than the number of satellites of a foreign system necessary to determine the above mentioned offsets and scaling parameters are used in the position solution, is very desirable.
Enabling the use of other satellites for positioning usually increases the number of satellites in view by a user, so that the user can select the satellites used to give optimal performance, as described in patent application DE 10 2007 006 612.2-35. Especially in areas for which a common view of four satellites of one system is hardly possible due to too many obstructions, the usable availability of satellites of other constellations will increase the probability of having more or even sufficient sources of navigation signals available.
To reduce the complexity of an inter-system combined positioning at user receiver level several features of the design of the systems are needed. Some of these are already foreseen to be implemented by the current GPS and planned Galileo design.
The representation of the navigation parameters for Galileo is the same as those for GPS. The terrestrial reference system used as origin of the coordinate system is defined similar to that used by GPS in a way that the user is affected negligibly.
A common reference of the system time scales is a crucial point. Since an establishment of a common reference for the system time, however, contradicts the system independence, such a common reference to come into existence is very unlikely. Nevertheless, it is essential for an integrated positioning solution. To overcome this limitation, GPS and Galileo are going to disseminate the system time difference between both systems in their navigation messages (cf. IS-GPS-200D Sec. 30.3.3.8.1 and GAL-OS-SIS-ICD/D.0 Sec. 10.1.2.7).
However, this solves only a part of the problem since only one inter-system parameter is addressed (system time offset). Other system differences like the different terrestrial reference systems and algorithms used to compute the navigation parameters are not covered but affect the accuracy and especially the integrity of the user's positioning solution.
The augmentation systems EGNOS and WAAS distribute differential corrections for GPS. These can be used to improve the accuracy and integrity of GPS-based positions within the limited service areas of these augmentation systems. The corrections are partly (for the ionosphere) disseminated not referring to a specific satellite but are mapped to a grid on the Earth's surface. Orbital and temporal corrections for every navigation satellite are provided by the data dissemination satellites of the respective augmentation system. This has the drawback that a high bandwidth is necessary to disseminate the information for all satellites in the service area within one data stream.
The dissemination of the GPS-Galileo system time difference enables a basic interoperability of the systems for the common user of the open service, where no service guarantees are provided. The professional user will most likely not benefit from this basic interoperability because he relies on a guaranteed reliability of services (accuracy, integrity). Since the knowledge of the quality of navigation parameters provided by a foreign system for the foreign system is usually very limited by the operator of the own system, no service guarantees can, thus, be provided to those professional users for a navigation solution incorporating also measurements to foreign satellites.
For some situations, the monitoring network and the processing facilities of a GNSS need to be capable of handling measurements of foreign GNSS. For GNSS receivers used in the ground monitoring network, this is nowadays usually the case and some GNSS of the prior art record the ranging signals and the navigation messages of foreign GNSS. However, these GNSS do not use the recorded data for further processing.
Combined GPS/GLONASS receivers are already on the market, and combined GPS/Galileo receivers are under development. GPS/Galileo receivers will soon be available and are foreseen to be used in the Galileo monitoring network. Receiver technologies integrating the capability of all three systems can be expected soon and further GNSS are assumed to be implemented quickly in the future. Hence, receiver technology is not a limiting factor.
The basics of satellite navigation used by different GNSS (ranging code concepts, frequencies used, satellite constellations) are very similar and so are the user processing algorithms.
For the algorithms to derive the ephemeris and time corrections this is not necessarily the case. GPS, for example, relies on Kalman filters to determine the orbits and clocks, whereas Galileo uses batch least square fits to solve the same problem. In GLONASS the Earth orientation parameters are estimated within the control segment, whereas GPS uses the predictions provided by IERS Paris.